1. Field of the Invention
The present invention relates to a driving method for a recording head for thermal recording by selective heat generation in plural heat generating elements in response to heat generating signals, and to a thermal printer utlizing said driving method.
The above-mentioned driving method for the thermal head and the thermal printer utilizing said driving method are adapted for use in a recording apparatus such as a printer for computer output, an electronic typewriter, a copying machine, a facsimile machine or the like.
2. Related Background Art
Rapid progress in the information industry in recent years has stimulated development of various information processing systems, with suitable recording methods and apparatus therefor. Among such recording methods there is already known a thermal recording method, which is recently widely used due to the compactness and light weight of the apparatus, absence of noise in operation, excellent operability and ease of maintenance.
Such thermal recording method is classified into thermal-sensitive recording and thermal transfer recording. The former uses a thermo-sensitive recording sheet, containing a color generating material and a color developing material and capable of generating a color, as the recording medium, and forms a recorded image by heat signals given by a recording head having an array, on a substrate, of plural heat generating elements capable of generating heat by electric power supply. The latter uses a thermal transfer material having, on a substrate film, a thermally transferable coating composed of a coloring material dispersed in a heat fusible binder, and forms a recorded image by overlaying said thermal transfer material with a recording medium such as paper, with said thermal transfer ink coating in contact with said recording medium and supplying heat signals by said recording head across the substrate of said thermal transfer material, thereby fusing and transferring said thermal transfer ink onto said recording medium.
However, such thermal recording methods, both thermal-sensitive recording and thermal transfer recording, may be associated with the following concern. In selective activation of the heat generating elements according to the image information, if constant energy is given to said elements regardless of the types of image information, there may result uneven density in the recorded image, due to the difference in cooling and heat accumulation inside the substrate and glaze of the recording head depending on the nature of the image information.
More specifically, the temperature or accumulated heat in the recording head immediately before activation of the heat generating element is different in a case of activating the heat generating element after absence of image information for a while in the scanning direction of the recording head, than in a case of continuously activating the element in response to continuous image information, so that the temperature realized by the heat from said element also becomes different.
Consequently, if the electric energy supplied to the heat generating element is maintained always constant, there may result unevenness in the density in an area composed of a large number of dots, or a large color-developed area of the thermo-sensitive sheet or a large ink-transfer area of the thermal transfer ink.
Also in the thermal transfer recording, when the recorded image is formed as a film which does not penetrate into the recording medium and can be lifted off for erasing an erroneous record, the ink may be fused excessively in a large transfer area due to the accumulated heat thus causing diffusion into the recording medium and resulting in an uneven erasure in such lift-off operation.